1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to method and system for dimmer output emulation.
2. Description of the Related Art
Electronic systems utilize dimmers to direct modification of output power to a load. For example, in a lighting system, dimmers provide an input signal to a lighting system. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers use a digital or analog coded dimming signal that indicates a desired dimming level. For example, some analog based dimmers utilize a triode for alternating current (“triac”) device to modulate a phase angle of each cycle of an alternating current (“AC”) supply voltage. “Modulating the phase angle” of the supply voltage is also commonly referred to as “chopping” the supply voltage. Chopping the supply voltage causes the voltage supplied to a lighting system to rapidly turn “ON” and “OFF” thereby controlling the energy provided to a lighting system.
FIG. 1 depicts a lighting system 100 that includes a triac-based dimmer 102. FIG. 2 depicts exemplary voltage graphs 200 associated with the lighting system 100. Referring to FIGS. 1 and 2, the lighting system 100 receives an AC supply voltage VSUPPLY from voltage supply 104. The supply voltage VSUPPLY is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. Triac 106 acts as voltage-driven switch, and a gate terminal 108 of triac 106 controls current flow between the first terminal 110 and the second terminal 112. A gate voltage VG on the gate terminal 108 will cause the triac 106 to turn ON and current iDIM when the gate voltage VG reaches a firing threshold voltage value VF and a voltage potential exists across the first and second terminals 110 and 112. The dimmer output voltage Vφ—DIM is zero volts from the beginning of each of half cycles 202 and 204 at respective times t0 and t2 until the gate voltage VG reaches the firing threshold voltage value VF. Dimmer output voltage Vφ—DIM represents the output voltage of dimmer 102. During timer period TOFF, the dimmer 102 chops the supply voltage VSUPPLY so that the dimmer output voltage Vφ—DIM remains at zero volts during time period TOFF. At time t1, the gate voltage VG reaches the firing threshold value VF, and triac 106 begins conducting. Once triac 106 turns ON, the dimmer voltage Vφ—DIM tracks the supply voltage VSUPPLY during time period TON. Once triac 106 turns ON, triac 106 continues to conduct current iDIM regardless of the value of the gate voltage VG as long as the current iDIM remains above a holding current value HC. The holding current value HC is a function of the physical characteristics of the triac 106. Once the current iDIM drops below the holding current value HC, i.e. iDIM<HC, triac 106 turns OFF, i.e. stops conducting, until the gate voltage VG again reaches the firing threshold value VF. The holding current value HC is generally low enough so that, ideally, the current iDIM drops below the holding current value HC when the supply voltage VSUPPLY is approximately zero volts near the end of the half cycle 202 at time t2.
The variable resistor 114 in series with the parallel connected resistor 116 and capacitor 118 form a timing circuit 115 to control the time t1 at which the gate voltage VG reaches the firing threshold value VF. Increasing the resistance of variable resistor 114 increases the time TOFF, and decreasing the resistance of variable resistor 114 decreases the time TOFF. The resistance value of the variable resistor 114 effectively sets a dimming value for lamp 122. Diac 119 provides current flow into the gate terminal 108 of triac 106. The dimmer 102 also includes an inductor choke 120 to smooth the dimmer output voltage Vφ—DIM. Triac-based dimmer 102 also includes a capacitor 121 connected across triac 106 and inductor 120 to reduce electro-magnetic interference.
Ideally, modulating the phase angle of the dimmer output voltage Vφ—DIM effectively turns the lamp 122 OFF during time period TOFF and ON during time period TON for each half cycle of the supply voltage VSUPPLY. Thus, ideally, the dimmer 102 effectively controls the average energy supplied to the lamp 122 in accordance with the dimmer output voltage Vφ—DIM.
The triac-based dimmer 102 adequately functions in many circumstances. However, when the lamp 122 draws a small amount of current iDIM, the current iDIM can prematurely drop below the holding current value HC before the supply voltage VSUPPLY reaches approximately zero volts. When the current iDIM prematurely drops below the holding current value HC, the dimmer 102 prematurely shuts down, and the dimmer voltage Vφ—DIM will prematurely drop to zero. When the dimmer voltage Vφ—DIM prematurely drops to zero, the dimmer voltage Vφ—DIM does not reflect the intended dimming value as set by the resistance value of variable resistor 114. For example, when the current iDIM drops below the holding current value HC at time t3 for the dimmer voltage Vφ—DIM 206, the ON time period TON prematurely ends at time earlier than t2, such as time t3, instead of ending at time t2, thereby decreasing the amount of energy delivered to lamp 122. Thus, the energy delivered to lamp 122 will not match the dimming level corresponding to the dimmer voltage Vφ—DIM.